The India Meteorological Department issued its signal on Tuesday in its daily national weather bulletin. “Conditions are becoming favourable for onset of southwest monsoon likely over parts of south Bay of Bengal, Andaman Sea and Andaman and Nicobar Islands towards the end of this week,” the department stated.
The low-pressure area over the southwest Bay of Bengal, first identified on Monday, remained active on Tuesday. The IMD said the associated cyclonic circulation extends up to 4.5 kilometres above mean sea level and will intensify further over the next 48 hours.
How Early Is It?
The climatological normal date for monsoon onset over the Andaman Sea region is around May 22. An arrival by the end of this week would place the onset five to six days ahead of schedule.
Last year, in 2025, the monsoon reached the Andaman Sea on May 13. This year’s projected arrival falls within a similar window, continuing a pattern of early onset that forecasters have observed in recent seasons.
The monsoon then advances westward toward mainland India. Its arrival over Kerala on June 1 is the date that formally marks the start of the four-month season. That date remains the national benchmark.
An early arrival over the Andaman Sea does not guarantee an early onset over Kerala, nor does it predict how much rain the country will receive. The IMD has consistently noted that the timing of the initial onset has no direct link to the monsoon’s subsequent progress or the total quantum of seasonal rainfall.
The southwest monsoon delivers over 70 per cent of India’s annual rainfall during the June to September period. The country’s average seasonal rainfall stands at approximately 880 millimetres. What matters for agriculture, reservoirs, and groundwater recharge is not when the monsoon begins, but how consistently and widely it delivers rainfall through its four-month course.
Weather System Driving the Early Onset
The trigger for this week’s expected onset is the low-pressure area sitting over the southwest Bay of Bengal. Such systems draw in moisture-laden winds from the Indian Ocean and create the atmospheric conditions that allow the monsoon to advance northward and westward.
The IMD noted that a trough in the easterlies runs from the centre of this low-pressure area through to Marathwada in the lower troposphere. This pattern, combined with the strengthening cyclonic circulation, is what forecasters are reading as a favourable signal.
A fresh Western Disturbance is also expected to affect northwest India from May 15, adding a secondary weather driver to an already active national weather picture.
Once the monsoon establishes itself over the Andaman Sea, forecasters will track its northward advance toward the Bay of Bengal and then westward toward the Kerala coast. The IMD will issue further bulletins as the system develops.
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